With the proliferation of electronic or electric devices, each operating on a variety of voltage supplies and currents, used or integrated in a vehicle, the electric load of the vehicle becomes larger. A complicated electric power system of the vehicle is also needed to accommodate these devices. This results in a power supply system of low efficiency. Besides, it can be difficult to further improve the fuel efficiency and to reduce waste of energy for a conventional vehicle with, for example, a single 12 V electric system, which can lead to poor performance and high energy loss to accommodate these devices. In addition, as more of these devices are connected to the electric power system of the vehicle, hidden danger of fault increases, which reduces the reliability of the electric power system. Therefore, there is a need for a power management system to co-ordinate the operation of the electric power supply to these devices, to improve the reliability and efficiency of the electric power supply.